well, just running the math, you'd be going about 64.4 mph at 3000 rpm if your tire diameter was 23". That seems like a reasonable, cruising speed for a four cylinder and I personally wouldn't go any shorter on the gearing. I hope your speedo isn't telling you 75mph at 3000 rpm. Then again, that could keep you out of trouble

(3000rpm/.75*4.25)*(23in*pi)*(60)/(5280*12)=64.39984941 mph

It's pretty simple; I just used your gear ratios to figure out the rpm at the front wheel and then I converted that into inches per minute using your tire diamater. Lastly, inches per minute is converted into mph.